The fibre beside the Romania–Georgia cable: what it could hear in the Black Sea, and in whose waters

How an optical fibre hears#

A telecom cable carries light. At the shore end, a box called an interrogator can be attached. It sends short laser pulses into the fibre and listens to the light that comes back. Microscopic variations in the glass scatter a small fraction of the light back from every point. When a vibration stretches the fibre by a few billionths, the phase of that light shifts, and the interrogator learns where and by how much. The method is called distributed acoustic sensing (DAS). On the cable between the Netherlands and England, the fibre was read every 1.021 m; for the analysis, the researchers kept 19,583 channels about 4 m apart.

The limit is the returning light. The record reported by 2022 was 171 km in a controlled experiment on low-loss fibre; on the North Sea cable, the data were usable for the first 80 km. Long cables, though, have repeaters, amplifiers placed every 40–100 km. They let light pass one way only. Beyond the first repeater, fine listening from shore stops.

Beyond the first repeater there are other methods. In 2022 British researchers measured, from the shore, spans of a 5,860 km transatlantic cable, up to 12 repeaters at a time, using return paths that cable's repeaters already had, without changes to the underwater infrastructure. The result is one reading per 45–90 km span, not thousands of points. That is enough for earthquakes and ocean swell; for ships it has not yet been shown. A third method uses sensors built into the repeaters themselves, which the UK proposal names explicitly.

What cables have already heard#

On the 209 km cable from Zandvoort in the Netherlands to Leiston in Suffolk, Wageningen researchers listened to the first 80 km in 2026. They found ship noise between kilometre 0.5 and kilometre 2 of the fibre, where the cable crosses a shipping lane, a vessel passing near kilometre 0.6 whose noise showed up to 100 Hz, and the noise of a wind farm under construction. Turbines already running were not heard. In a separate analysis of the whole cable they saw a controlled explosion that the Dutch meteorological institute reported about 100 km off the coast; in the fibre's data it appears 120 km from shore.

Off Svalbard, a fibre of about 120 km located a cargo ship around 86.5 km of FO cable, with a root-mean-square deviation of about 50 m from the positions the ship broadcast by AIS, and caught whale calls out to at least 95 km along the cable (Landrø et al., 2022). On the Curie cable from Los Angeles to Valparaíso, the polarisation of the light flagged about 20 earthquakes in nine months, among them a magnitude 7.7 off Jamaica; there, the entire length of a submarine cable acts as a single sensor (Zhan et al., 2021).

Two things are missing from the literature. No published study shows a submarine detected by a fibre. And no peer-reviewed study gives the greatest distance, to either side of the cable, at which a ship can be heard. One study not yet peer-reviewed, on a cable off Zeebrugge, tested thresholds from 0.5 to 5 km without establishing a maximum range. The same study cites bottom trawlers detected up to 2.5 km from the cable (Wienecke and Brenne, 2023) and, in a bibliography note, an Alcatel Submarine Networks paper for the SubOptic conference reporting Large vessels up to 8 km away from the cable. That is why, on the map, the reader chooses the distance.

Listening to the sea through cables is an old idea. From 1952 the US Navy laid lines of hydrophones on the ocean floor, wired to shore stations, the SOSUS system, to track Soviet submarines; after the Cold War scientists were given access to the recordings and recognised whale song in them. What is new is that the sensor is the commercial cable itself, and its data belong to a company.

What Britain proposes#

On 22 October 2026 the British government published a White Paper on the security of critical undersea infrastructure (CP 1683), out for public consultation until 14 January 2027. It proposes penalties of Up to 14 years' imprisonment for intentional conduct. Chapter 6 is about something else: the cable as a sensor. If accessed by an adversary, sensing data could potentially compromise UK defence interests.

• Who would answer for it. the entity that controls the fibre on which the sensing technology is deployed, even when the data are collected, processed or stored by someone else.
• What it would tell the state. Who owns the cable and who runs the sensor; what kind of sensor, its capabilities and intended use, the data format (raw or processed), sample rate, spatial resolution, frequency range and data retention plans; where it sits, including the location of any interrogator units.
• When. At least six months ahead on an existing cable; at the planning stage for a new one, typically at least 12 months before installation begins; no later than within 90 days of the relevant provisions entering into law for systems already running. For locating a fault: an advance provisional notice, then an activation notice of at least 72 hours.
• What the state could do. Impose conditions and, rarely, prohibit a deployment. Responsible entities would not need to wait for the government to finish its review before proceeding with a deployment, except under an interim order.
• The data. Entities would comply with government requests for data generated by the sensing technology, through mechanisms for sharing it in the form and at the frequency requested (consultation question 60).
• Power cables too. The proposed regime would cover fibre integrated into subsea data and energy conveyance systems, plus sensors integrated into cable repeaters.

The unusual part is the boundary. The proposal would cover sensors on cables that land in Britain if they can detect activity in the UK's territorial sea, exclusive economic zone or continental shelf. It would also cover subsea cables that pass through UK waters but do not directly connect to the UK mainland (question 66). The government's reasoning: a sensing deployment cannot sensibly be divided by reference to maritime boundaries, and such a sensor is equally capable of revealing UK military patterns of activity. The British test asks what a sensor can hear, wherever its box sits.

None of this is law yet. The White Paper does not say the government already receives data from commercial sensors and names no existing system. Another part of the document, directions to operators during an incident (preserving evidence, rerouting traffic, delaying a repair), with fines of up to £17 million or 10% of global turnover (whichever is higher), would apply to data cables only: We do not propose including providers of subsea energy conveyance systems or services within scope.

The context is military. On 22 January 2025 the Defence Secretary, John Healey, told the Commons that the Russian vessel Yantar is a Russian spy ship used for gathering intelligence and mapping the UK's critical underwater infrastructure. On 9 April 2026 he announced that British forces and allies had tracked submarines of Russia's deep-sea research directorate (GUGI), saying: They are designed to survey underwater infrastructure during peacetime, and sabotage it in conflict.

The European Union recommends similar measures, without imposing comparable obligations through these documents. Its Action Plan on Cable Security (21 February 2025) says cable systems can be used as large geographical sensor networks to monitor nearby activities, for civilian and military purposes, and the toolbox published on 5 February 2026 recommends acoustic sensing on existing and future commercial cables. The same toolbox proposes a data cable between the South Caucasus and Bulgaria and Romania.

The Black Sea cable, kilometre by kilometre#

The Black Sea Submarine Cable would link Topraisar, in Romania's Constanța county, with Anaklia in Georgia: 1,300 MW of direct current at ±525 kV over about 1,155 km, about 1,115 km of it under water, according to a statement from Transelectrica, Romania's grid operator, on 30 July 2026. The promoters are Transelectrica and Georgia's grid company, GSE. The EU made it a Project of Mutual Interest in December 2025, and the seabed survey that will confirm the optimal route of the cable is now being prepared. No final investment decision has been taken.

The fibre would not sit inside the power cable. GSE writes: Integration of TLC fibers inside the power HVDC cables system is not possible due to the presence of repeaters and water depth constraints. Instead, A repeated submarine fiber optic cable is needed due to the length of the link, with repeaters 70–100 km apart and up to 24 fibre pairs. Its stated purpose is commercial: internet between Europe, the Caucasus and Central Asia. No published document mentions using it as a sensor.

The line on the map is the reference route for the seabed survey, published by GSE in October 2025 and redrawn here on coordinates; in cross-validation the mean error was about 1.3 km, so the line's position and the distances are approximate. It is 1,125 km long and crosses the economic zones of four states: 170 km in Romania's, 56 km in Bulgaria's, 708 km in Turkey's and 191 km in Georgia's. Its deepest point is 2,203 m, at km 479. Transelectrica's statement says some segments would lie at depths of over 2,500 metres; neither the redrawn line nor GSE's published profile goes that deep.

The Black Sea has no high seas: beyond internal waters and territorial seas, all of it is divided into exclusive economic zones. The line passes about 10.1 km (8–12 km, allowing for the tracing error) from the zone off Crimea claimed by both Ukraine and Russia (around km 335), about 12.5 km (10–14 km) from Russia's zone (around km 898) and, near Anaklia, 1–5 km from the waters off Abkhazia, the breakaway region held with Russian backing. On its shore at Ochamchire, 36 km from the line, in October 2023 the president of Abkhazia, a Russian-backed separatist region of Georgia, announced that a new Russian navy base would be built.

The ships that pass the route crowd towards its ends. The two stretches near shore, between 12% and 18% of the length, hold between 22.5% and 61% of the ship passages near the line and between 43% and 94% of the hours ships spend there, depending on how it is measured (2023 AIS data, EMODnet). Ships without AIS transponders, warships among them, are not in these data.

When a cable is damaged in another state's zone, who judges#

The waters a cable lies in matter beyond sensing. The exclusive economic zone gives the coastal state rights over resources, not full jurisdiction: the UN Convention on the Law of the Sea leaves every state free to lay cables there, and for an incident of navigation, trying the crew falls in principle to the flag state or the state of the sailors' nationality. Recent cases show how open the question is.

1. 7–8 October 2023 · Gulf of Finland The Balticconnector gas pipeline and two data cables, torn by the anchor of the NewNew Polar Bear. In August 2024 China acknowledged the ship's responsibility, as an accident; Finland and Estonia said the Chinese report does not qualify as official evidence.
2. 17–18 November 2024 · Sweden's zone The C-Lion1 and Sweden–Lithuania cables. Swedish investigators found that It cannot be determined with certainty whether a Chinese ship intentionally damaged data cables; Chinese authorities kept jurisdiction over the Yi Peng 3.
3. 25 December 2024 · Finland's zone The Estlink 2 power cable between Finland and Estonia, and four data cables, damaged by the anchor of the tanker Eagle S. On 3 October 2025 the Helsinki district court dismissed the charges: the acts took place outside Finnish territorial waters. On 27 August 2026 the Court of Appeal ruled that Finland does have jurisdiction, as the events directly impacted the country. The ruling can still go to the Supreme Court.
4. 25 February 2025 · Taiwan The Taiwan–Penghu cable, cut off Tainan. The captain of the Hong Tai 58 was sentenced by the Tainan court on 12 June 2025 to three years in prison.
5. 8 July 2026 · Black Sea Romania, Bulgaria and Turkey widen their 2024 mine-countermeasures group, to the protection of critical undersea infrastructure.

In the Eagle S case, both where the consequences occurred and how the acts were classified mattered: the Court of Appeal did not treat them as a navigation incident under the convention. The British sensing rule asks a related one: what a sensor can detect, and in whose waters. On the Black Sea, where no water belongs to nobody, either answer points to at least two states.

What has not been decided#

For the Black Sea cable, the questions can be asked now, while it is still on paper.

• Who would control the fibre? The promoters are Transelectrica and GSE; who will own and run the data cable has not been announced.
• What kind of listening would be possible? A shore interrogator would listen finely only to the first ~100 km from each end, Romanian and Georgian waters, with Bulgarian waters 20–24 km away, Turkish waters 16–20 km away and the waters off Abkhazia 1–5 km from the line. Span readings or sensors in repeaters would reach the whole length, through four states' zones.
• Which rules would apply? Romania requires authorisation for marine scientific research in its economic zone (Law 17/1990, art. 39), and official hydrographic data belong to the Defence Ministry's Maritime Hydrographic Directorate (Law 395/2004). As a transmission operator, Transelectrica falls under Romania's cyber-security rules (emergency ordinance 155/2024, which transposes the EU's NIS2 directive) and the critical-entities law (Law 294/2024). None of them mentions data from a cable used as a sensor. The cable industry's committee, the ICPC, argues that sensing done to protect a cable belongs to the right to lay cables, not to scientific research; the UN convention does not define marine scientific research.
• Where would the data be kept, and who could ask for them? The British proposal follows the data wherever the data is held. The sources reviewed show no comparable dedicated regime for this project; existing rules would apply depending on the activity.